The invention relates generally to a machining device and particularly to a portable apparatus for machining the surfaces of anode contact buttons employed in Hall cells, though the invention is not limited thereto.
In Hall cells employed for the production of aluminum, a plurality of large carbon anodes are suspended in the electrolyte of each cell by elongated copper bars secured to a large bus located over the cell. Between the bus and the copper bars, and welded to the face of the bus, are metal buttons that are employed to conduct current from the bus to the rod and hence to the anode at the lower end of the rod. Such buttons in an operating cell become pitted and/or oxidized due to arcing between the button and the contacting surface of the anode bar. When this occurs, a substantial voltage drop develops between the pitted or oxidized surface of the buttons and the contacting surface of the anode bar. Any voltage drop, of course, lowers the voltage across the cell, and hence the current flow through the cell, such that the efficiency of the cell in producing metal is adversely affected.
Heretofore, in order to face such buttons, the particular cell had to be isolated electrically from the circuit of adjacent cells so that personnel could work safely on the cell to reface the buttons. The voltage across any particular Hall cell is quite small, i.e., on the order of four or five volts. However, the potential (voltage) of the cell above ground is substantial such that it is potentially dangerous for personnel to work on the cells while the cells are operating.